In the existing communication systems, Digital Subscriber Line (xDSL) is a technique for high speed data transmission over telephone twisted pairs (e.g. Unshielded Twist Pair, UTP). In addition to the base-band transmission DSLs, such as Integrated service digital network (ISDN) DSL (IDSL) and High speed DSL (HDSL), the pass-band transmission xDSL utilizes the frequency division multiplexing (FDM) technique so that the xDSL service may coexist with the POTS (Plain Old Telephone Service) in a same twist pair. The xDSL service occupies the higher frequency band, while the POTS occupies the base band below 4 KHz. As shown in FIG. 1, the POTS signal and the xDSL signal may be separated from each other by a splitter. A system that provides multiple xDSL access is called a DSL Access Multiplexer (DSLAM).
Because xDSL signals are transmitted over UTP which is originally designed for voice transmission, there may be a lot of factors damaging the high frequency signals, such as external interference, noise, interference between conductors including in the same cable, and line parameter alterations due to environmental variations. These factors may cause instable operation of an xDSL system.
After years of development, the xDSL technique has evolved, from the first generation, i.e. Asymmetric Digital Subscriber Line (ADSL), into the current second generation, i.e. ADSL2, ADSL2+ and Very High Speed Digital Subscriber Line 2 (VDSL2), with the bandwidth and the number of frequency bands increasing gradually. ADSL and ADSL2 utilize a downlink bandwidth below 1.1 MHz, providing downlink rates of up to 8 Mbps. In ADSL2+, the downlink bandwidth is expanded to 2.2 MHz, providing a downlink rates of up to 24 Mbps. VDSL2 utilizes a bandwidth of up to 30 MHz, providing rates of up to 100 Mbps simultaneously in both the uplink and downlink directions. With the bandwidth expanding in xDSL technique, the crosstalk in high frequency bands is becoming increasingly significant.
As shown in FIG. 2 and FIG. 3, near end crosstalk does not cause much damage to the performance of an xDSL system, because frequency division multiplexing is utilized for uplink and downlink channels in xDSL. However, far end crosstalk has a significant impact on the performance of transmission lines. In other words, when multiple users request to put services into operation over a bundle of cables, some lines may be of low rate, instable performance or even can not be put into operation because of the far end crosstalk, resulting in a lower DSLAM line activation rate. In the application scenario as shown in FIG. 3, a much severer crosstalk may be resulted.
In view of the above, many operators have defined their own spectrum application management specifications for specifying spectrum planning under various application scenarios, to prevent performance deterioration due to mutual interferences between devices of various locations.
Currently, carriers of some frequency bands may be switched off to prevent crosstalk. As shown in FIG. 4, carriers, that are overlapped with ADSL downlink frequency bands, in far end ADSL2+ are switched off, to reduce downlink interference from the carriers to ADSL of Central Office (CO). By switching off the carriers, the crosstalk from downlink signals of a remote module to the central office module may be prevented, because the bandwidths are no longer overlapped.
In the above method of switching off carriers in frequency bands, the requirements of spectrum management of spectrum compatibility may be met. However, as shown in FIG. 4, only carriers in frequency band above 1.1 MHz are utilized in the far end DSLAM (i.e. the remote module), and attenuation in the frequency band above 1.1 MHz is greater than that in a frequency band below 1.1 MHz. Therefore, the transmission performance may be deteriorated rapidly with the lengthening of transmission distance, thereby bringing about a significant limitation to the performance of the remote module.
In addition, because a fixed spectrum setting is employed in the above method of switching off carriers, a dynamic adaptive variation according to the spectrum variations in lines is impossible, resulting in low spectrum utilization.
Furthermore, in the above method of switching off carriers, the corresponding spectrum is required to be configured manually. In other words, an automatic configuration is impossible in the above method. This brings about a large workload of manual configuration in the case of complicated lines.